Tissue hypoxia is frequently seen in critically ill patients and it perhaps predisposes these patients to development of multiple system organ failure. In cellular terms, hypoxia is characterized by decreases in the intracellular concentration of oxygen, leading to a decline in aerobically produced adenosine triphosphate (ATP). The deficit arising from unequal levels of cellular ATP requirements and aerobic ATP production is partially satisfied by anaerobic sources of ATP, including glycolysis, the creatine kinase reaction, and the adenylate kinase reaction. These reactions can set in motion cellular mechanisms that ultimately may lead to cellular dysfunction and death. A clear understanding of the relative importance of these reactions is impossible to acquire from global measures of oxygen delivery and oxygen consumption; therefore, the clinical monitoring of tissue oxygenation also should include the measurement of metabolically relevant, organ-specific variables.